It is known to stabilize synthetic-resin tubes upon their extrusion from a die by sealing the same to maintain a superatmospheric pressure in the space between the die and the sealed portion to support the otherwise flacid tube wall until solidification, hardening, curing, or cooling of this wall has been completed.
As is known in the art and has been described in "EXTRUDIEREN VON PROFILEN UND ROHREN" (VD1-Verlag 1974 S. 98, 99), the conventional seals or plugs for the interiors of extruded tubes can be connected to the region of the extrusion die by a tractive element which enables the tube to pass by the plug. Such systems have considerable disadvantages, mainly because of the significant wear to which the seal or plug is subject. This is especially the case when large-bore or large-caliber pipe is made at extrusion speeds between 3 and 15 cm/minute.
To reduce the wear in this manner, it is known to provide a plurality of such plugs connected one after the other.
Another way of sealing the interior of the tube is to squeeze the same shut at a location remote from the extrusion die. This process is only effective if pipes of limited length are to be made and has been found to severely limit the production or output because the squeezed portion must be cut from the tube and discarded. The squeezing process itself is time consuming and labor consuming.